The need to accurately, reliably and easily determine a patient's blood pressure has long been recognized as a goal. Typically, the blood pressure varies with time in a series of irregularly shaped pulses. The diastolic pressure is the baseline pressure for these pulses. The systolic pressure is defined to be the peak pressure for these pulses. Another useful measure of the pressure is the mean arterial pressure, that is, the time weighted average of the pulses. Ordinarily, the mean arterial pressure is not the arithmetic average of the systolic and diastolic pressures, but rather, is lower than the arithmetic average because the pulses tend to be weighted towards the bases than the peaks.
Various methods have been known to the art for determining blood pressure. Broadly, measurement methods may be categorized as either intrusive or non-intrusive. Intrusive blood pressure measuring devices are inserted directly into the blood vessel to make blood pressure measurements. While this method provides accurate blood pressure measurements, it tends to be somewhat painful and requires the attention of a skilled operator.
Non-invasive methods can provide useful measures of the blood pressure with no intrusion into the body. In the auscultatory method, a pressurizable cuff is placed around the upper arm of the patient and inflated to a pressure above the systolic pressure. This stops the flow of blood in the underlying artery. The cuff pressure is slowly decreased and a stethoscope or microphone is used to listen for Korotokoff sounds which accompany the resumption of blood flow in the artery. The systolic pressure is taken to be the cuff pressure at which the sounds are first heard and the diastolic pressure is taken to be the cuff pressure at which the sounds disappear. The auscultatory method provides an easy, inexpensive way to determine the systolic and diastolic pressure of a patient. This method tends not to be very precise, and does not directly provide a measure of the mean arterial pressure.
Another noninvasive method is termed the oscillometric method. The oscillometric method monitors the oscillations in pressure in an inflatable cuff which are caused by variations in the arterial pressure. FIG. 1 shows a graphic representation of a normal oscillation pulse 10. The relative magnitude of the oscillations in cuff pressure are used to estimate the blood pressure. Parameters of the oscillations 10 considered relevant by the prior art included the pulse peak points 12, the pulse base 14 pressure and the lower peak 16 pressure. In the method known as the `Base to Peak` method, the pressure differential between the pressure pulse peak 12 and the pressure at the pulse base 14 (labeled h.sub.1 and h.sub.2) was measured in a variety of prior art methods. (see e.g., the note here to fill in prior art.) Ordinarily, this pressure differential would be measured at a variety of cuff pressures. The cuff pressure could be changed either continuously or incrementally. The mean arterial pressure is usually taken to be the cuff pressure at which the oscillation magnitude is greatest. For example, in the base to peak method, that point would be when the pressure differential between the pulse peak 12 pressure and pulse base 14 pressure is maximized with respect to cuff pressure. The systolic pressure is usually taken to be the cuff pressure above mean arterial pressure at which the oscillation magnitude is a fixed percentage or fraction of the oscillation magnitude at mean arterial pressure. The diastolic pressure is usually taken to be the pressure below the mean arterial pressure at which the oscillation magnitude has dropped to a fixed percentage or fraction of the oscillation magnitude at the mean arterial pressure.
An alternative measurement of the oscillation 10 in cuff pressure is termed the `Peak to Peak` value. In this method, the pressure differential between the value at the pulse peak 12 and the lower peak 16 is determined. Those pressure differentials are labeled as a.sub.1 for the first cycle shown in FIG. 1B, and a.sub.2 for the second cycle shown. See, e.g., U.S. Pat. Nos. 4,360,029 and 4,394,034 by Maynard Ramsey, III, and 4,461,266 by Hood.
The reliability and repeatability of these methods hinges on their ability to accurately determine oscillation magnitude. There are several barriers to accurate and reliable oscillation magnitude determination. First, artifacts caused by patient motion and other effects are nearly always present. These artifacts are superimposed upon the desired oscillation signal, causing it to be distorted. Second, many of the properties of the desired oscillation signal are not consistent from patient to patient, or even from oscillation to oscillation for a given patient. One factor which affects the consistency of these properties would include irregular heart rate.
The prior art methods which follow the Base to Peak or Peak to Peak methods have employed a variety of schemes to improve the accuracy and the reliability of the methods. Most often, the schemes involve artifact detection and rejection. Examples of artifact rejection algorithms can be seen for example in the U.S. Pat. Nos. 4,360,029 and 4,394,034 by Maynard Ramsey, III (artifact rejection algorithms look at, inter alia, select parameters such as peak height or time rate of change of successive samples or series of samples), 4,546,775 by Medero (rejection is signal slope is uncharacteristic of true complex). These techniques will accept only pulses with certain properties, such as specific rise times, or certain consistencies, such as consistent time between oscillations. While these techniques may work well in some cases, they may fail in other cases. Such artifact rejection schemes tend not to work well with very old or very ill patients, as such properties or consistencies may simply not be present. In these cases, these prior methods can yield unreliable measurements of blood pressure or no measurement at all.
Another disadvantage of the prior techniques is that the artifact rejection procedures often require a series of measurements to determine whether consistency is present. The more repetition of tests or measurements which are necessary increases the overall time for the blood pressure determination.